Electroluminescent lamp with boric acid or boric oxide used in conjunction with the zinc sulphide layer



Nov. 15, 1966 v P w. RANBY ETAL 3,286,115

ELECTROLUMINESCENTIAMP WITH BORIC ACID OR BORIC OXIDE USED IN CONJUNCTION WITH THE ZINC SULPHIDE LAYER Filed Feb. 24, 1964 PETER WHITTEN RANBY DAVID WlLLlAM SMITH //vvEA/T0S United States Patent Ofllice 3,286,115 Patented Nov. 15, 1966 3,286,115 ELECTROLUMINESCENT LAMP WITH BORIC ACID R BORIC OXIDE USED IN CONJUNC- TION WITH THE ZINC SULPHIDE LAYER Peter Whitten Ranby and David William Smith, London,

England, assignors to Thorn Electrical Industries Limited, London, England, a British company Filed Feb. 24, 1964, Ser. No. 346,605 Claims priority, application Great Britain, Feb. 25, 1963, 7,534/ 63 11 Claims. (Cl. 313108) This invention relates to organic electroluminescent devices, that is, electroluminescent devices in which the phosphor is dispersed in an organic medium, and in particular to flexible organic electroluminescent devices, in which the electroluminescent cell or cells are not supported on a rigid base, but are enveloped in a flexible envelope which is light-transmitting over at least a part of its surface.

It is an object of the present invention to provide a flexible organic electroluminescent device showing improved maintenance of light output.

According to the invention there is provided a flexible organic electroluminescent device in which a layer of an electroluminescent material of the zinc sulphide type dispersed in an organic medium contains and/ or is in contact with boric acid or boric oxide. Devices according to the invention not only exhibit superior maintenance of light output after long use but also show less discolor-ation in use.

Electroluminescent materials of the zinc sulphide type include, apart from zinc sulphide itself, related phosphors such as zinc cadmium sulphide, zinc selenide and zinc sulpho-selenide.

The amount of boric acid or oxide used is preferably between 0. 25 and 30%, and is advantageously between and 15% of the weight of the dry phosphor. Where the device contains a layer of dielectric material disposed between the phosphor layer and an opaque electrode, the boric acid or oxide can be incorporated in the dielectric layer.

In another embodiment of the invention, the boric acid or oxide is included as a separate film between the phosphor layer and a light-transmitting electrode.

When the boric acid or oxide is to be incorporated in the phosphor layer it can be added to the phosphor before fabricating the lamp by griding and sieving and then mixing thoroughly with the phosphor. Alternatively a solution of the boric acid or oxide can be added to the phosphor powder which is then dried, ground and sieved.

The invention will now he further described with reference to the following specific examples of the construction of a flexible electroluminescent device, taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a flexible electroluminescent device, of the type described in copending application No. 176,373 and FIG. 2 is a section along the lines IIII of FIG. 1.

Referring to FIG. 1, there is shown a number of individual electroluminescent cells 2 arranged end to end over continuous strip current conveyors 4, 4, the whole assembly being sealed in an envelope 6 of transparent plastic. The envelope 6 is sealed at '8 between the individual cells, and the connectors 10, 12 in each cell 2 connect the current conveyors 4, 4 with the electrodes of the cell.

In the particular construction shown in FIG. 2 the envelope 6 is of transparent plastic film, such as polythene about 0.015 inch thick or Kel-F film about 0.005 inch thick, and the current conveyor 4 and connector (and the conveyor 4 and connector 12, not shown) are of flexible copper strip or ribbon. Each cell comprises a layer 14 of a phosphor in a binder, such as cyanoethylcellulose or a vinylidene copolymer, sandwiched between a transparent conducting electrode 16 and a base electrode 18 of aluminum coated with a dielectric layer 20 of barium titanate dispersed in a cyanoethylcellulose or vinylidene copolymer binder. The transparent electrode 16 is of micro-fibre glass paper, rendered conductive by treatment such as described in application No. 176,373. The layer 22 is of polythene or nylon, which on heating fuses to bond the electrode 16 to the phosphor layer 14 and to the connector 10.

Example 1 A quantity of a powdered copper-activated zinc sulphide phosphor was intimately mixed with 10% by weight of boric acid of analytical reagent grade which had previously been finely ground and sieved through a 250 mesh silk sieve. The phosphor/boric acid mixture was then used in the construction of an electroluminescent device as described above. Similar devices were constructed in which the quantities of boric used were respectively 0%, 7 /2%, 12 /2%, 15% and 20% of the weight of the dry phosphor powder.

These lamps were run continuously and their respective brightnesses compared after various intervals, with the following results, 100 being taken as the initial brightness of the boric acid free lamp:

Percent of H3BO3 Hours of continuousrunning Example 2 A powdered zinc sulphide phosphor, finely ground and sieved through a 250 mesh silk sieve was intimately mixed with 8% by weight of boric oxide of analytical reagent grade. The mixture obtained was used in the construction of a flexible electroluminescent device as described in Example 1. The resulting device showed a light output maintenance similar to that of the device employing 10% by weight of boric acid as shown in the above table.

What is claimed is:

1. An electroluminescent device comprising a hermetically sealed flexible envelope at least a portion of which is light-transmitting, and an electroluminescent cell embedded in said envelope, said cell comprising a pair of spaced electrodes superposed in face to face relationship at least one of said electrodes being light-transmitting, means for conducting current to each of said electrodes, and a layer of a zinc sulfide electroluminescent phosphor dispersed in an organic binder sandwiched between said electrodes, said zinc sulphide being substantially in contact with a material consisting essentially of a member selected from the group consisting of boric .acid and boric oxide.

2. An electroluminescent device comprising .a hermetically sealed flexible envelope at least a portion of which is light-transmitting, and an electroluminescent cell embedded in said envelope, said cell comprising a pair of spaced electrodes superposed in face to face relationship at least one of. said electrodes being light-transmitting, means for conducting current to each of said electrodes, and an electroluminescent layer of zinc sulfide and a material consisting essentially of a member selected from the group consisting of boric acid and boric oxide uniformly dispersed in an organic binder layer sandwiched between said electrodes.

3. An electroluminescent device according to claim 2, wherein said layer contains from about 0.25% to about 30% by weight of said material based on the dry weight of the zinc sulphide.

4. An electroluminescent device according to claim 2, wherein said layer contains from about 5% to about 15% 'by weight of said material based on the dry Weight of the zinc sulphide.

5. An electroluminescent device comprising a hermetically sealed flexible envelope at least a port-ion of which is light-transmitting, and an electroluminescent cell embedded in said envelope, said cell comprising a pair of spaced electrodes superposed in vface to face relationship at lea-st one of said electrodes being light-transmitting, means for conducting current to each of said electrodes, and a pair of layers sandwiched between said electrodes in face to face contact, the first of said layer-s comprising a zinc sulfide electroluminescent phosphor substantially uniformly dispersed in an organic binder and the second of said layers comprising a material consisting essentially of a member selected from the group consisting of boric acid and boric oxide.

6. An electroluminescent device according to claim 5, wherein said second layer contains from about 0.25% to about 30% by weight of said material based on the dry weight of the zinc sulphide.

7. An electroluminescent device according to claim 5, wherein said second layer contains from about 5% to about 15% by weight of said material based on the dry weigh-t of the zinc sulphide.

8. An electroluminescent device comprising .a hermetically sealed flexible envelope at least a portion of which is light-transmitting, and an electroluminescent cell embedded in said envelope, said cell comprising .a light-trans rnitting electrode and an opaque electrode superposed in spaced face to face relationship, means for conducting current to each of said electrodes, and two layers sandwiched between said electrodes in face to face contact, the first of said layers being disposed between said opaque electrode and the second of said layers and comprising a member selected from the group consisting of boric acid and boric oxide substantially uniformly dispersed in a dielectric material and the second of said layers comprising a zinc sulfide electroluminescent phosphor substantially uniformly dispersed in an organic binder.

9. An electroluminescent device according to claim 8, wherein said first layer contains from about 0.25% to about 30% by weight of said material based on the dry weight of the zinc sulphide.

10. An electroluminescent device according to claim 8, wherein said first layer contains from about 5% to about 15% by weight of said material based on the dry weight of the zinc sulphide.

11. A laminated electroluminescent device comprising an envelope of thermoplastic material at least a portion of which is light-transmitting, at least two electroluminescent cells hermetically sealed within the envelope and insulated from each other by the hermetic seal of said envelope, said cells comprising at least two superposed electrodes, at least one of which is light-transmitting, and a layer of light-emitting material including a zinc sulfide electroluminescent phosphor disposed between said electrodes, said electroluminescent material consisting essentially of at least one member being substantially in contact with a material selected from the group consisting of boric acid and boric oxide; means to conduct current to each of said electrodes in each of said electro1umi nescent ce l-ls, said means comprising at least two longitudinally disposed current conveying means, insulated from each other, and each insulated from one of the electrodes in said cells, said current conveying means extending along the length of said electroluminescent device and passing across the seal between each of said elec troluminescent cells.

References Cited by the Examiner UNITED STATES PATENTS 2,941,103 6/1960 Nagy et al. 313-108 2,944,177 7/1960 Piper 313-108 2,945,128 7/ 1960 Kuan-Han Sun et al. 250- 3,161,797 12/1964 Butler et al. 313-108 JAMES W. LAWRENCE, Primary Examiner. R. JUDD, Assistant Examiner. 

1. AN ELECTROLUMINESCENT DEVICE COMPRISING A HERMETICALLY SEALED FLEXIBLE ENVELOPE AT LEAST A PORTION OF WHICH IS LIGHT-TRANSMITTING, AND AN ELECTROLUMINESCENT CELL EMBEDDED IN SAID ENVELOPE, SAID CELL COMPRISING A PAIR OF SPACED ELECTRODES SUPERPOSED IN FACE TO FACE RELATIONSHIP AT LEAST ONE OF SAID ELECRODES BEING LIGHT-TRANSMITTING, MEANS FOR CONDUCTING CURRENT TO EACH OF SAID ELECTRODES, AND A LAYER OF A ZINC SULPHIDE BEING SUBSTANTIALLY IN CONDISPERSED IN AN ORGANIC BINDER SANDWICHED BETWEEN SAID ELECTRODES, SAID ZINC SULPHIDE BEING SUBSTANTIALLY IN CONTACT WITH A MATERIAL CONSISTING ESSENTIALLY OF A NUMBER SELECTED FROM THE GROUP CONSISTING OF BORIC ACID AND BORIC OXIDE. 